Hillary Clinton, Donald Trump, “Suicide Squad’’ character Harley Quinn, and clowns are among the most popular Halloween costumes this year. But former Celtics player Paul Pierce is opting for something old school. Pierce, who’s playing his final NBA season with the Los Angeles Clippers, posted a picture of himself on Instagram the other day dressed as Rick James, the Jheri-curled authorof “Give It to Me Baby’’ and “Super Freak.’’ “I’m Rick James You kno wat I’m saying,’’ Pierce wrote.